Body mass index

From Ganfyd

Click here to use the ganfyd Body mass index calculator.

Abbreviated to BMI. Weight (in kilograms) divided by height (metres) squared. BMI is also known as Quetelet (body mass) index.^[1] It is used as a surrogate measure of adiposity. You can expect excess mortality if you weigh too little or too much.^[2] The ideal range in adults is 20-25 kg/m², though the downside of its simplicity is that the measurements are less useful in certain groups (see Limitations). The relationship between BMI and other, not truely independent variables, such as smoking, sex race or socio-economic group has been studied in many populations with an overview emphasising that the BMI is very useful in understanding the contribution under nutrition and over nutrition has to mortality and morbidity.

• Under 20 classed as underweight
• Over 25 classed as overweight
• Over 30 classed as obese
• Over 40 classed as morbidly obese

As smoking decreases in Western populations this leaves obesity as the most important lifestyle risk factor able to be changed by simple decisions that will enhance lifespan for an individual.

Limitations

• Large muscular bodybuilder may have the same BMI as overweight sedentary office worker.
• Less useful in certain ethnic groups (e.g. South-east Asians). The accepted limits are lower in Asian. ^[3]
• In children, calculated the same way, but classification based on centile.

BMI and Specific Conditions

Death rate for given BMI in different age cohorts^[4]

BMI and Mortality

A classic and expected J shaped curve of absolute risk exists with age increasing the risk, particularly at the extremes of weight. However in never smokers there is far less relative risk associated with being below the ideal BMI and there is an increasing relative risk of death as the BMI increases from a nadir of about 25. So much so that males with a BMI more than 35 are at more risk of death than females who smoke, but are at their optimal BMI.^[5] This is important because as smoking decreases in Western populations obesity becomes the most important risk factor that will increase lifespan, able to be changed by simple lifestyle decisions of an individual.

BMI and Coronary Heart Disease Risk

Several studies suggest that BMI per se is not a good predictor of cardiovascular morbidity and mortality.^[6][7][8]

• EBM Waist hip ratio independently predicted death from cardiovascular and coronary heart disease in Australian men and women ^[9]
• NLH Q&A Service. Should we move to measuring abdominal girth rather than BMI when assessing cardiovascular risk factors, is it a better predictor?
• NLH Q&A Service. Recently waist hip ratio has been shown to be a risk factor for CHD - What are the Quintiles to which the risk relates and how much is the risk in each quintile?

Raised BMI and contraception

• For a woman with a body mass index of 40 kg/m², is combined oral contraception (COC) a safe and effective method of contraception? (From Faculty of Family Planning and Reproductive Health Care (FFPRHC).)
• For women with a high body mass index (BMI), which contraceptive methods are safe and effective? (From FFPRHC.)
• Should women with a body mass index of 43 kg/m² who use progestogen-only pills take two pills per day to improve contraceptive efficacy? (From FFPRHC.)

Internet BMI calculators

Links from this page can not be checked for compliance against medical device law applicable to stand alone software in relevant jurisdictions: In Europe a medical algorithm or clinical decision assistance tool implemented in software (eg an app) that serves a therapeutic purpose such as calculating a drug dose, if made generally available or marketed is regarded as a medical device As of 2012 the FDA is consulting on a similar requirement

• Ganfyd's own BMI calculator
• From the US National Heart, Lung, and Blood Institute
• from the US Centers for Disease Control and Prevention
• from the BBC

References